The present invention relates to hydraulic systems and pertains particularly to a dual pressure system with means for ensuring a separation of the pressure within the system.
Some hydraulic systems employ different pressures for different portions of the system. For example, some motors of the system may be designed to operate at much higher pressures than other motors of the system. The higher-pressure motors are required to perform functions that require the higher pressures, whereas the lower-pressure motors perform certain functions that do not require such high pressures. Accordingly, such systems are normally designed such that the fluid for the system is supplied by a single pump or source of fluid with dual-stage relief valves for establishing the higher and lower stage pressures.
Such systems are normally designed such that the lower pressure prevails when the lower-pressure motors are being operated and the high pressure prevails when the higher-pressure motors are being operated. This is normally accomplished by dual-stage relief valves which are normally set at the lower-stage pressure but is responsive to directional control valves of the higher-pressure motors for directing the fluid for controlling the multistage relief valve when the higher-pressure motors are in operation. Such systems must also be arranged such that the higher-pressure motors and the lower-pressure motors are not required to be operated at the same time. Such systems, which have this arrangement, can be supplied from a single pump or a single series of pumps with a common supply and with a multistage relief valve as discussed above.
An example of a system employing such dual-stage relief valves is shown, for example, in U.S. Pat. No. 3,831,620, issued Aug. 27, 1974 to Donald L. Bianchetta et al and assigned to the assignee hereof. The above-described system also discloses such a dual-stage pressure relief valve. A high-pressure portion of such a high pressure or multistage system is also disclosed in U.S. Pat. No. 3,774,707, issued Nov. 27, 1973 to John W. Bridwell et al and similarly assigned to the assignee hereof. Such systems normally employ shuttle valves between the respective high-pressure control valves of the high-pressure system and the relief valve for permitting a selected one of the control valves to communicate with the relief valve for establishing the higher pressure of the system. A simple shuttle-type or ball-type shuttle valve such as 106 in the above-mentioned Bianchetta patent is normally employed.
Such valves, however, have been found to be unsatisfactory in certain applications such as, for example, wherein the valve is employed between the forward and reverse drive of the vehicle. Such dual-stage hydraulic systems may be found, for example, on hydraulic excavators. Such excavators may employ, for example, a high-pressure system on the order of 4400 psi for operating the track drive of the vehicle and a lower pressure on the order of 3600 psi for operation of the implements of the vehicle. With such systems, the higher pressure is necessary in the drive system in order to obtain sufficient power to move the vehicle, however, it is not necessary that the implements be operated at the much higher pressure. For this reason, the implement system is not normally designed for the higher pressures. Such higher pressures, therefore, may cause failure of components of the lower-pressure system such as the conduits and the like which communicate the fluid to the respective motors.
Systems such as described above have been found to have been operated at the higher pressures by operators who have learned to override the relief valve simply by depressing a pair of the control valves of the high-pressure system simultaneously to cancel each other out and at the same time to increase the relief valve to the higher pressure stage. This is accomplished by simultaneous depression of the forward and reverse control valves. The higher pressure and speeds resulting from this operation can cause early failure of the components of the system.